feat: add pause resume billing adjustment

[eischideraa-unn]

Summary

This PR implements subscription pause/resume billing adjustment support for the app’s subscription lifecycle. It adds the missing billing behavior around paused subscriptions so users can pause an active subscription, receive a prorated billing credit, and resume without losing billing continuity. The fix ensures the subscription state, pause history, and next billing date behave consistently during pause and resume cycles.

Closes #932

Motivation

The codebase already had pause-related UI/state plumbing, but the billing adjustment flow was incomplete and inconsistent across the subscription store. Specifically, the app needed a clear and safe flow for:

• pausing an active subscription
• calculating prorated credit based on the pause duration
• tracking the active pause state and credit remaining
• resuming a subscription and shifting the next billing date appropriately
• preserving pause history for auditing and UI display

This PR closes that gap in the key subscription logic implemented in subscriptionStore.ts and the pause model in pause.ts.

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What Changed

1) Subscription pause logic

The store now properly creates a pause record when a subscription is paused:

• marks the subscription as inactive
• records the pause reason and note
• calculates credit using the subscription price and pause duration
• stores billing adjustment metadata and scheduled resume date
• keeps pause history available for later lookup and UI display

Implemented in:

• subscriptionStore.ts

2) Billing adjustment metadata

The pause record model now includes the fields needed to represent billing-adjustment behavior and resume lifecycle tracking, including:

• credit amount
• credit remaining
• billing adjustment alias
• planned resume date
• resume status
• pause lifecycle state

Updated in:

• pause.ts

3) Resume flow with billing restart

When a subscription is resumed:

• the active pause is located and resumed
• the subscription is set back to active
• the remaining credit is preserved as part of the resume record
• the next billing date is shifted forward by the pause window to maintain billing continuity

This protects against billing drift when a subscription is paused and later resumed.

4) Regression coverage

Added targeted unit tests covering the critical pause/resume billing flow, including:

• pause creates credit-bearing pause record
• subscription is marked inactive while paused
• resume reactivates the subscription
• next billing date advances after resume

Test coverage added in:

• subscriptionPauseResume.test.ts

5) Documentation alignment

The issue flow is consistent with the pause/resume billing documentation already present in:

• PAUSE_RESUME.md

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Edge Cases Considered

• Prevents invalid pause creation when a subscription is already paused
• Handles missing or malformed pause history safely
• Uses existing store conventions and persisted state handling
• Keeps billing state aligned with the UI’s pause/resume expectations
• Preserves historical data for downstream analytics and review

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Testing

Added focused unit coverage for the newly introduced behavior:

• pause creates a structured pause record with prorated credit
• resume marks the subscription active again
• next billing date is adjusted after resume
• pause history reflects resume lifecycle changes

Note: this PR is prepared with the implementation in place and unit tests added, but the full CI quality gates below remain to be validated in the project environment.

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Pull Request Checklist

Quality Gates (All must pass before merge)

• Lint: Code passes ESLint and Prettier checks
• Type Check: TypeScript compilation succeeds
• Tests: All tests pass
• Build: Project builds successfully
• Rust Format: Smart contract formatting is correct
• Rust Clippy: Smart contract linting passes
• Rust Tests: All smart contract tests pass
• Rust Build: Smart contracts compile successfully

Additional Requirements

• New code has appropriate TypeScript types
• No hardcoded secrets or credentials
• New features have corresponding tests
• Documentation updated if needed

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